This invention relates to the manufacture of tubular sections of ductile materials and, more particularly, to a novel method of forming bends or of changing wall eccentricity by pushing the tubular section through a tilted die causing a greater diameter reduction on one portion of the tube circumference than on the opposite portion.
Numerous bending processes have been developed over the years, but generally speaking, most such methods are variations of a few basic processes. No single process can be successfully applied to all bending situations where variations of tubular section size, diameter-to-wall thickness ratio, material or angle of bend are considered. For instance, the press method, wherein the tube is laid across a plurality of wiper dies and then subjected to the pressure exerted by a form die, is useful when some flattening of the tubing can be permitted. The roll method of bending employs three or more triangularly arranged rolls, the center one of which is adjustable. The workpiece is fed between the fixed driven rolls and the adjustable roll to form the bend. The draw method bends the tube by clamping it against a rotating form and drawing it through a pressure die. In all of these methods, thinning of the tube wall, especially on the extrados, and loss of section circularity occur. The thinner the tube wall and/or the tighter the bend sections, the more severe these problems become.
In attempting to eliminate loss of cross section circularity, the use of various types of mandrels or other means of internal support has been employed with varying degrees of success. In some instances, the use of internal tools has led to process complications or given birth to new problems such as scarring of the inner wall or non-uniform wall thinning.
U.S. Pat. No. 3,354,681 discloses a method and apparatus for bend-forming elbows from tubular sections by pushing through a forming die. A portion of this apparatus consists of a "tapered land" which the inventor claims to cause bending by differential friction, the friction force being greater on the inside radius of the bent tubular section than on the outside radius, which is in direct contradiction to the findings of our invention.
Another problem pervasive in the tubing industry is that of tube wall eccentricity. Eccentricity may be loosely defined as the distance between the center of the tube cross section with respect to its inner diameter and the center with respect to its outer diameter. When such centers do not coincide, the member is eccentric. Eccentricity correction is concerned with reducing differences in wall thickness. U.S. Pat. No. 3,095,083 discloses a method and apparatus for correcting eccentricity by drawing (pulling) the member through a tilted die without the use of internal tools. However, not only is the amount of eccentricity correction obtainable limited but it has been found that the die will in some instances produce wall thickening and in other instances produce wall thinning. This same technique to effect eccentricity correction is employed in U.S. Pat. No. 3,131,803 wherein the tilted die is used in combination with an internal mandrel. Other approaches to eccentricity correction are also employed, for example: U.S. Pat. No. 3,167,176 uses a swivel mandrel, and U.S. Pat. No. 3,698,229 uses metal removal from the heavy wall portion of the tube.